1. Technical Field
Improved cooling systems with high-efficiency condensers are disclosed which provide improved performance at low ambient temperatures. Improved methods of operating cooling systems with high-efficiency condensers at low ambient temperatures are also disclosed.
2. Description of the Related Art
As shown in FIG. 1, large commercial cooling systems like the one shown at 10 generally include an evaporator 11, an accumulator 12, one or more compressors 13, one or more condensers 14 and a throttling device or expansion valve 15. The system 10 illustrated in FIG. 1 is a dual system with one refrigerant circuit 11, 13, 14, 15 shown at the left in FIG. 1 and a corresponding refrigerant circuit 11a, 13a, 14a, 15a shown at the right in FIG. 1. Referring to the refrigerant circuit 11, 13, 15 shown at the left in FIG. 1, refrigerant flows through the continuous refrigerant loop 19 of the refrigerant circuit 11, 13, 14, 15. A heat transfer fluid is circulated through heat transfer tubing 16 in the evaporator 11 to transfer heat from the heat transfer fluid to refrigerant passing through the evaporator 11. Alternatively, heat may be transferred from the air in a climate controlled area to the refrigerant in the evaporator 11 by means of a forced air process. The heat transfer fluid chilled in the evaporator tubing 16 is normally water or glycol, which is circulated to a remote location to satisfy a cooling load. The refrigerant in the evaporator 11 evaporates as it absorbs heat from the heat transfer fluid, and the compressors 13 operate to extract and compress this refrigerant vapor, and to discharge the compressed vapor to the condenser 14. In the condenser 14, the refrigerant vapor is condensed and the liquid refrigerant is delivered back to the evaporator 11 through the throttling device 15, where the refrigerant cycle begins again.
There is an increasing demand for energy efficient cooling systems. In the system 10 illustrated in FIG. 1, system capacity is gained by employing multiple compressors 13. At lower ambient temperatures, only one or perhaps two of the three compressors 13 are utilized. Further, at lower ambient temperatures, only one of the two refrigerant circuits 11, 13, 14, 15 or 11a, 13a, 14a, 15a are utilized. System efficiency is also typically gained by adding more surface area to the condensers 14, 14a. 
Still referring to the refrigerant circuit 11, 13, 14, 15 shown at the left in FIG. 1, the combined surface area provided by the large condenser coil surface areas 17, 18 increases efficiency of the system 10 at high ambient temperatures, by lowering the discharge pressure of compressor 13, thus lowering the electricity consumed by compressor 13. This same concept also applies when the ambient temperature is low. Specifically, when a demand for air conditioning is made while the ambient temperature is low, the discharge pressure from the compressors 13 is too low, even with only one compressor 13 operating and the refrigerant cycle 11a, 13a, 14a, 15a shown at the right in FIG. 1 turned off. As a result, operation of the system 10 at low ambient temperatures cause the compressor 13 in the system to run outside of its safe operating range as the combination of low ambient temperatures and the high-efficiency condenser 14 design results in a great amount of heat being removed from the refrigerant cycle 11, 13, 14, 15 and discharged to the atmosphere which, in turn, results in lower than optimal discharge pressures at the lone compressor 13 that is operating. On one hand, unit software or low pressure switch may prevent the compressor 13 or system 10 from running at low ambient temperature conditions, to the dismay of the user. On the other hand, if the system 10 does operate at low ambient temperatures, compressor 13 failure may occur, also to the dismay of the user.
One way to operate the system 10 safely at low ambient temperature conditions is to lower airflow across the condenser 14, which reduces the heat removal through the condenser 14 thereby increasing discharge pressure to a safer level at the compressor 13. Therefore, in order to operate the system 10 at low ambient temperature conditions, variable speed motors 21, 22 need to be installed to control the speed of the fans 23, 24, which is expensive, labor intensive and requires a more complicated control system (not shown).
Accordingly, improved methods for operating cooling systems at low ambient temperatures and improved cooling systems systems that operate safely and efficiently at low ambient temperatures are desired.